Seawater desalination is one of the methods for solving the global scarcity of freshwater resources, usually including a thermal method and a membrane method. In which, the membrane seawater desalination technology using the reverse osmosis membrane is widely used due to the low cost and power consumption, and gradually becomes predominant.
In the seawater desalination system, the preprocessed seawater having a lower salt content and entering the desalination system is raw seawater, which forms high-pressure raw seawater after being pressurized. One part of the high-pressure raw seawater passes through the reverse osmosis membrane module and becomes low-pressure freshwater, the other part is high-pressure condensed brine which is turned into low-pressure condensed brine after releasing pressure energy through the energy recovery device and then discharged.
The technology includes three core components: a reverse osmosis membrane, a high-pressure pump and an energy recovery device. The high-pressure pump increases the pressure of the raw seawater to 5-7 Mpa so that the freshwater of a proportion of about 40% passes through the reverse osmosis membrane. The remained condensed brine of a proportion of about 60% still has a pressure potential energy of about 6 Mpa, and it shall be transferred into the raw seawater through the energy recovery system to reduce the total energy consumption.
The key of the technology is how to reduce the investment cost, operation cost and energy consumption of the high-pressure pump and the energy recovery device. The sum of the costs of the high-pressure pump and the energy recovery device is about ⅓ of the total investment cost, their power consumptions are more than ⅔ of the total power consumption, and the power consumption costs occupy more than ⅓ of the operation cost. At present, the power consumption of freshwater generated in the method is 3-5 kwh/ton, while the limit power consumption under the current technological level of the membrane shall be 2.5 kwh/t, and there is still ⅓ energy saving space.
Currently, there are two types of high-pressure sea water pumps for seawater desalination: one is a piston type, which converts the power of motor rotation into a linear motion of the piston in the columnar cylinder block through a crank-link mechanism to pressure seawater; the structure achieves a high efficiency and the pump efficiency is more than 80%; but the flow is not stable enough and the pressure fluctuation is obvious; It is controlled by valves and the flow quantity is limited by the length of the crank link, thus the flow direction change is frequent, the vibration and noise are large, and the fault rates of the control valve and the sealing element are high. The other is a centrifugal pump which increases the water pressure through a centrifugal force generated by multi-stage rotor rotation; the flow is high and stable without requiring any valve control; but the efficiency is lower, and the pump efficiency is usually below 80% and averagely about 75%. Since the seawater is highly corrosive and lowly viscous, the support and flow passage components of the two types of pumps both shall be made of high-quality anti-corrosion and wear resistant material, such as copper alloy, duplex steel and even ceramic material, and their costs are very high.
Currently, there are also two types of energy recovery devices for seawater desalination: one is based on the principle of water turbine, wherein the high-pressure condensed brine drives the turbine to rotate to pressurize the raw seawater, without requiring a flow distribution control or a booster pump, and the flow is stable and continuous; But it needs two conversions, i.e., concentrated seawater pressure potential energy→shaft rotation mechanical energy→raw seawater pressure potential energy, thus the recovery efficiency is low and usually 60%, which has been gradually rejected by the industry. The other is based on the principle of pressure exchange, i.e., in the columnar cylinder block, the high-pressure condensed brine directly transfers the pressure potential energy to the raw seawater through a flow distribution mechanism; the transfer efficiency is very high and the energy recovery efficiency is more than 90%; Depending on the flow distribution manner, there is a rotary cylinder block with end port flow distribution structure without a piston energy recovery device and a fixed cylinder block valve controlled flow distribution structure with or without a piston energy recovery device; As for the rotary cylinder block without a piston energy recovery device (e.g., the PX series products of an American company), it is a simple structure but having a 2-5% mixing between the raw seawater and the condensed brine, and an independent booster pump is needed. That decrease the total efficiency; As for the fixed cylinder block valve controlled flow distribution structure with a piston, it does not require a booster bump and the efficiency is a little higher, but the control mechanism is complex. Relevant patents and practice at home and abroad are all based on the above technical solutions.
Therefore, based on many years of relevant design and manufacturing experiences, the inventor proposes a membrane seawater desalination pressurization and energy recovery integrated method and device to overcome the defects of the prior art.